The present invention is related to a method of driving a matrix liquid crystal display, particularly a high capacity display device.
For flat panel displays, liquid crystals can be used as pictures elements (pixels). These pixels are arranged in a matrix and each pixel can be actuated through a switch, typically implemented with a thin film transistor TFT). The switch is turned on by means of two-dimensional X-Y addressing such as that used in a random-access memory.
A typical block diagram is shown in FIG. 1(A). In this figure, the pixels, such as P11 and P21, are located at the cross-points of an X-Y matrix. The matrix of the liquid crystal display panel has n rows in the X-direction and m columns in the Y-direction. Hence, there are mXn TFTs, such as 1a, as well as liquid display elements, such as 1b. The TFTs function as switches for actuating the liquid crystal pixels. The scanning electrodes (the gates) of the TFTs in the same row are connected together and driven from drivers with outputs G1, G2, . . . , Gm. The input terminals of the switches (say, the sources) in the same column are connected together and fed with pulsed information or data signals.
FIG. 1(B) shows the scanning waveforms in different parts of a conventional system with labels corresponding to that in FIG. 1(A). The pulsed waveforms G1, G2, G3, G4 are successively delayed by one dwell time of a horizontal line, which is equal to the horizontal scan time. These waveforms are applied to the rows G1, G2, . . . , Gm respectively to control the gates of the TFTs. In this manner, the TFTs are sequentially turned on for information signals to be impressed on the corresponding liquid crystals.
When the TFT is turned on, the information or data voltages are impressed on the liquid crystals for display. These voltages stay with the corresponding liquid crystals until the signal voltage is reset or inverted when no signal of the same color is applied to the liquid crystals.
In the foregoing description, the scanning bus G1, G2, . . . , Gm in FIG. 1(A) have voltage waveforms shown in FIG. 1(B). Under ideal condition, this waveform is not distorted or delayed, and the system should perform well. In actual conditions, each TFT has finite on resistance and the liquid crystal is a capacitive element. As a result, there is a finite charging and discharging time for the picture elements to reach the desired signal voltage. Since the dwell time of the signal for each pixel is very short, the pixel may not have enough time to be charged up to the desired signal voltage, causing the display to darken.
Tekeda etal disclosed in U.S. Pat. No. 4,651,148 a method to overcome this problem by not only charging the addressed pixel but also precharging the following pixel simultaneously. The precharging can shorten the time for the addressed pixel to attain its final voltage. Precharging is effected either by using a longer addressing pulse than the dwell time of pixel or by using double pulses, one for precharging and the other for charging the liquid crystal to its final value. The first version is to lengthen the row control pulses to double the duration of the dwell time as shown in FIG. 2, G1, G2, G3, G4 waveforms. Note that G2 overlaps with G1 for one dwell time.
In another version, double pulses are used for precharging a and charging. FIG. 3 shows the waveforms at different points of Tekeda's double pulse system. The scan pulses are applied twice as shown in waveforms G1, G2, G3, G4, which are applied to the (i-3)th through (i)th row electrodes, whereas D1 shows the data signal waveforms for three colors, R, G, B, applied to the (j)th column electrode addressed. Compared to the conventional drive waveform D1, the drive waveform P11' substantially expands the scan pulse width by preliminary charging the electrode with data signals fed from the same color row that precedes the (n)th row. Waveform P11' shows the potential of the display picture electrodes in the (i)th row and the (j)th column. V.sub.i-n and V.sub.i respectively indicate the data voltages dealing with the (i-n)th row and the (i)th row. In the beginning of each field, each picture element remains charged in a reversed polarity by the preceding field. Next, when the switching transistor turns on, the display picture element electrode in the (i)th row and the (j)th column start the preliminary charge against the data voltage V.sub.i-n that precedes the (n)th row. The switching transistor then turns off during H.sub.i-n+1 through H.sub.i-n periods and again turns on during the next H.sub.i period, thus activating charge against the data voltage V.sub.i. As a result, a charge curve such as that shown in P11' is achieved, allowing these electrodes to charge voltages to such a level higher than the conventional drive method shown in P11. When the data signals V.sub.i-n and V.sub.i contain the same colors as in the TV pictures and have a relationship close to each other, the Tekeda drive method then provides the same effect as if the RC time constant were reduced.
The Tekeda method, however, has some serious drawbacks. These drawbacks are due to the inversion of the same polarity voltage signal occurring in the same vertical scanning field and the overlapping of same color signals also occurring in the same field. This situation causes serious flickering and cross-talk problems.
In the Tekeda method, the signal of the same color is impressed on the liquid crystals only during every alternate field. As shown in FIG. 4(A), the signal is applied only during the first field when they are positive. The voltages at the liquid crystals reset to a negative voltage or inverted in during the second field. The absence of signal during the second field makes the signal flicker at a 1/30 rate instead of 1/60 rate. Thus the flickering effect is more pronounced.
The second drawback of the Tekeda system is that the overlapping of the pulses of the same color as shown in FIG. 3, waveforms G1 and G4. In both versions of the Tekeda method, the resultant signal voltage applied to the two neighboring pixels of the same color is indicated as P11 and P21 in FIG. 2. Note that in the middle interval when the driving pulses on G1 and G2 overlap, signals appear both in P11 and P21. Such an overlap of signals may cause cross-talk. This problem arises because the polarity of all the drive voltages such as P11, P21, etc are of the same polarity in the first field, before the polarity is inverted or reset in the second field as shown in FIG. 4(A). In other words, the Tekeda system only has field inversion, which is inadequate.